The invention relates to a method of producing a screen having a striped structure of electroluminescent material on a display window of a colour display tube, which method comprises exposing a photosensitive material on the display window to light emitted by a elongated light source and passed through a lens system and a shadow mask, which shadow mask is suspended from the display window and which lens system is positioned between the elongated light source and the shadow mask, the lens system, forming an image of the elongated light source on the screen, comprising a first element for substantially correcting the rotation of the image of the elongated light source and a second element for substantially determining the landing position.
The invention also relates to a color display tube provided with a screen which is produced using the method.
A method of producing a screen for a color display tube as described in the opening paragraph is disclosed in the United States patent specification U.S. Pat. No. 4,226,513. This specification describes an exposure device for making a striped structure on the display window of a color display tube. The method uses an elongated light source and two so-called correction lenses. The first one, closest to the light source, prevents the rotation of the image of the elongated light source and the second one takes care of the landing aspects.
In colour display tubes where the luminescent material is applied in a striped structure, these stripes tend to zigzag near the edges of the screen. This zigzag form is due to the way the elongated light source is imaged on the display window. In general the mask and/or the display window are not completely flat, so that the longitudinal axis of the elongated light source and the axis through the slit shaped aperture of the shadow mask are not parallel, causing a rotation of the image of the elongated light source on the screen. The first correction lens in the exposure system counteracts this rotation. Inherently, these correction lenses are symmetric with respect to the long and the short axis along the screen.
The second correction lens serves to make sure that the light emitted by the elongated light source during the exposure process hits the screen at the same positions as the electron beams during operation of the color display tube.
Another important parameter of the exposure process is the line-growth factor. This parameter gives the increase in line width on the screen when the dose from the light source is increased. The prior art exposure process has the disadvantage that this line-growth factor is not constant for all positions on the screen. Due to an adjustment in the exposure process by changing the dose from the elongated light source or due to tolerances in the production process, the distribution of the line width over the entire screen is adversely influenced. This causes a deterioration with respect to the luminance distribution and the front of screen performance of the color display tube becomes worse.
It is an object of the invention to overcome the disadvantage of the prior art method by providing a method of producing a screen which delivers a constant line-growth factor for the entire screen.
According to the invention, this object is achieved by means of a method which is characterized in that the lens system further comprises a third element which is provided with means for adjusting the rotation of the image of the elongated light source over the entire screen.
The invention is based on the insight that the line-growth factor on the screen can be adjusted by deliberately introducing a rotation of the image of the elongated light sourcexe2x80x94also referred to as the lamp rotationxe2x80x94for each position on the screen. By deliberately rotating the image of the elongated light source, the width of the phosphor lines on the screen is influenced because the microscopic light distribution changes. The microscopic light distribution is the shape of the light spot as it is imaged by the elongated light source through the apertures in the shadow mask onto the screen. This shape, amongst others, is responsible for the line-growth factor.
In a further embodiment, the third element comprises a lens breaking the four-quadrant symmetry.
In the prior art method the first element corrects the rotation of the elongated light source. Due to the symmetry of the system, the vertical line through the centrexe2x80x94the north-south axisxe2x80x94has no rotation. Furthermore, the rotations in the four quadrants are mirror-symmetric. According to the present invention, the lamp rotation should be adjustable for all positions on the screen, i.e. also for positions on the north-south axis. Then, the distribution of the lamp rotation over the screen has lost its four-quadrant symmetry. To achieve this the third element should comprise a lens that breaks the mirror symmetry
In a preferred embodiment, the first element and the third element are combined to form an integrated element.
When the first and the third element are integrated, it will be clear that the number of elements has not changed. This has the advantage that, because both the first and the third element are part of one and the same lens, no additional measures have to be taken in the exposure device to enable the implementation of the present invention. It is just a matter of replacing the xe2x80x98prior artxe2x80x99 element by the element according to the invention. So, integrating the first and the third element is a very cost-effective measure.
In a still further embodiment, the integrated element has a first side comprising both the first element and the third element.
This has the advantage that this optical element can be manufactured using the same production methods as for the manufacture of the first element according to the prior art.
The invention further relates to a color display tube provided with a screen which is produced using the method, and more particularly to a color display tube with a striped structure extending in a zigzag way in an area of the screen passing through the centre of the screen and extending parallel to the striped structure.
In prior art color-display tubes, the striped structure in the area around the north-south axis does not show any lamp-rotation, consequently, the presence of a zigzag in the striped structure in this area is a clear indication of the use of the method according to the invention.